Research On Structure Construction And Performance Modulation Of Novel Carbon-based Perovskite Solar Cells

As an emerging photovoltaics,the power conversion efficiency(PCE)of perovskite solar cells(PSCs)have increased from 9.7%in 2012 to 25.5%in 2021,which has attracted great interest from both academic and industrial fields.Typical PSCs use Au/Ag as the back electrode to collect photogenerated holes,but these precious metals are expensive and easily corroded by halogen ions,in turn deteriorating device’s stability.Carbon-based perovskite solar cells(CPSCs)use carbon materials as back electrode and show excellent stability.However,the kinetic energy loss caused by the carbon layer in traditional C-PSCs and its related interfaces in carrier extraction and transport is serious,resulting in a fact that the PCEs of most C-PSCs are usually unsatisfactory(before 2018,mostly less than 17%).Development of new device architectures,reduction of charge recombination,and improvement of charge collection capabilities are the fundamental ways to solve this problem regarding the poor performance of C-PSCs.To this end,this thesis starts from a new type of modular C-PSCs,by means of innovative design of the device structure and development of new materials to greatly reduce the kinetic energy loss during the collection process of photogenerated holes,resulting a significant improvement of the device performance,based on which bifacial C-PSCs and tandem solar cells have been obtained.The main research results are as follows.Firstly,an innovative modular C-PSC was designed with the following structure features:(i)the photoanode and back electrode were prepared independently,then,the modular C-PSCs was assembled by mechanical lamination of above two electrodes by using carbon electrode as interconnecting layer;(ii)the lateral transport of photogenerated holes mainly relies on the highly conductive substrate(such as TCO or metal foil),in turn getting rid of the thickness dependence of carbon layer,then the dynamic energy loss during charge transfer process can be significantly mitigated.Three typical carbon materials(carbon black,graphite sheet,graphene)were used as back electrode,and the evolution of interface contact resistance under mechanical pressure was systematically investigated.The plastic and elastic characteristics of the carbon layer on the interface electrical contact of the modular C-PSC was analyzed,and the dynamic characteristics of the photogenerated holes at the relevant interface(carbon/spiroOMeTAD or carbon/perovskite interface)were deeply studied.The results indicate that graphene electrode exhibit the lowest interface contact resistance and the best hole extraction ability.The PCE of graphene-based modular C-PSCs reached 18.65%,which was significantly higher than that of traditional C-PSCs.Under ambient atmosphere(40%～80%humidity),the device without encapsulation can still retain 90%of the initial PCE after aging at 85℃ for 1000 h,indicating excellent stability.In addition,for this novel modular C-PSCs,almost no degradation in PCE was observed after repeated disassembling and assembling for more than 500 cycles,indicating good structural reliability.It has the advantages of repeated disassembly and easy maintenance.Secondly,investigations were carried out to regulate the electronic properties of reduced graphene oxide(rGO)using atomically dispersed Ti atoms,which was then used as back electrode to improve the interfacial charge transfer and transport ability.By combining several advanced techniques,such as synchrotron-radiation-based X-ray absorption spectroscopy,spherical aberration correction transmission electron microscopy,density functional theory(DFT)theoretical calculation,chemical structure simulation and other methods,a well-defined coordination structure of Ti-O4-OH was analyzed,and its effect on the electronic structure and electrical properties of rGO were studied.The results show that the electronic structure can be well regulated by the Ti-O4-OH structure,thereby increasing work function of rGO and reducing the contact resistance between the carbon electrode and the hole transport layer.Modular C-PSCs based on this material achieved high PCEs up to 21.6%,which was significantly higher than previous reported C-PSCs.Furthermore,the device without encapsulation can retain 90%of its initial value after 1000 h when measured by tracking at the maximum power point under 1-sun illumination,demonstrating excellent stability.Finally,carbon nanotubes(CNT)were used to fabricate transparent electrodes and bifacial C-PSCs,which are expected to effectively solve the problem that the traditional C-PSCs cannot effectively exploit the light illumination from rear side of the cell,and increase the total output power per unit area of the device.Three CNTs with different structures and diameters were used as the connecting layer,and its influence on the photovoltaic performance of bifacial CPSCs was systematically investigated.The results show that single-wall CNTs exhibit smaller interface contact resistance and better light transmittance when used as the back electrode.When the device was illuminated from the front side,a high PCE up to 21.4%was achieved,and a PCE of 16.8%was obtained as the device was illuminated from the rear side.The bifacial C-PSCs achieved a power output of 24.0 mW cm-2 in natural reflected surroundings(20%of AM 1.5G irradiance from rear side)and 34.1 mW cm-2 in artificial reflecting surroundings(100%of AM 1.5G irradiance from rear side).More importantly,this bifacial C-PSCs was integrated with a CuInSe2(CIS)bottom cell(band gap 1.0 eV)to fabricate 4-terminal tandem solar cell,which achieved a PCE of 27.1%.